Inventions come in all shapes and sizes. Some are as simple as purple catsup. Others push the limits of quantum physics. The real measure of an invention is not just how well it works or how impressively it is engineered, but how it changes our lives.

Ginger

Inventor: Dean Kamen

What is "it"? for a week or so last January, that was the question on everybody's lips. After the Harvard Business School Press advanced $250,000 for a book about Ita top-secret project under development by one of the most accomplished inventors in the U.S.reams of newsprint were devoted to speculation about what It might be. The recipient of the advance, author Steve Kemper, gushed in his book proposal that Itcode-named Gingerwould revolutionize personal transportation, urban design and our daily lives. Apple Computer ceo Steve Jobs said It could be bigger than the PC. Everyone had a different theory: Ginger was cold fusion, a flying car, a whole lot of hot air.

So what is It? Ten months later, we still don't know for sure, and no one who does is talking, least of all inventor Dean Kamen, 49. The only public utterance from the denim-clad millionaire was an early statement that "the leaked proposal quoted several prominent technology leaders out of context, without their doubts, risks and maybes included." (He also reportedly cut off all contact with Kemper, although Kemper has denied this.) The project was promising, Kamen said, but the expectations generated by the press were "beyond whimsical."

Kamen, whose invention credits include a portable drug-infusion pump, a compact dialysis machine and a wheelchair that can climb stairs, has refused to say another word on the subject. When pressed recently by a persistent journalist, he took her by the shoulders and asked, "What part of I'm not going to talk about this' don't you understand?"

The best clue to Kamen's intentions may be the patent application he filed in late 2000 for a series of self-balancing "personal mobility vehicles." It's known that Kamen and his colleagues have been working for years on a clean, sealed-combustion Stirling engine that could run on any fuel, including hydrogen. The prevailing theory is that Ginger would combine Stirling technology with a stabilizing system pioneered in Kamen's stair-climbing wheelchair. (The wheelchair's code name, by the way, was Fred. Get it? Fred and Ginger.) The newest clues are the names of two websites registered by Kamen-controlled companies: mystirlingscooter.com and flywheels.com.